turbostat plugin: free the right variable

turbostat plugin: free the right variable

scan-build: Using '/usr/bin/clang-6.0' for static analysis
make  all-am
make[1]: Entering directory '/home/ruben/src/collectd'
  CC       src/turbostat.lo
src/turbostat.c:1292:5: warning: Argument to free() is the address of the global variable 'thread_delta', which is not memory allocated by malloc()
    sfree(threads);
    ^~~~~~~~~~~~~~
./src/daemon/common.h:41:5: note: expanded from macro 'sfree'
    free(ptr);                                                                 \
    ^~~~~~~~~
src/turbostat.c:1299:5: warning: Argument to free() is the address of the global variable 'core_delta', which is not memory allocated by malloc()
    sfree(cores);
    ^~~~~~~~~~~~
./src/daemon/common.h:41:5: note: expanded from macro 'sfree'
    free(ptr);                                                                 \
    ^~~~~~~~~
2 warnings generated.
  CCLD     turbostat.la

(cherry picked from commit 74eda959ddf653b7d20779d84f0a140e85376726)